[Xen-devel] [PATCH v2 for-4.11] x86/pv: Hide more EFER bits from PV guests

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

We don't advertise SVM in CPUID so a PV guest shouldn't be under the
impression that it can use SVM functionality, but despite this, it really
shouldn't see SVME set when reading EFER.

On Intel processors, 32bit PV guests don't see, and can't use SYSCALL.

Introduce EFER_KNOWN_MASK to whitelist the features Xen knows about, and use
this to clamp the guests view.

Take the opportunity to reuse the mask to simplify svm_vmcb_isvalid(), and
change "undefined" to "unknown" in the print message, as there is at least
EFER.TCE (Translation Cache Extension) defined but unknown to Xen.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Release-acked-by: Juergen Gross <jgross@xxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
CC: Brian Woods <brian.woods@xxxxxxx>

Arguably, this wants backporting to the stable trees, so should be considered
for 4.11 at this point.
---
 xen/arch/x86/hvm/svm/svmdebug.c |  5 ++---
 xen/arch/x86/pv/emul-priv-op.c  | 11 +++++++++--
 xen/include/asm-x86/msr-index.h |  3 +++
 3 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/hvm/svm/svmdebug.c b/xen/arch/x86/hvm/svm/svmdebug.c
index 6c215d1..d35e405 100644
--- a/xen/arch/x86/hvm/svm/svmdebug.c
+++ b/xen/arch/x86/hvm/svm/svmdebug.c
@@ -133,9 +133,8 @@ bool svm_vmcb_isvalid(const char *from, const struct 
vmcb_struct *vmcb,
         PRINTF("DR7: bits [63:32] are not zero (%#"PRIx64")\n",
                vmcb_get_dr7(vmcb));
 
-    if ( efer & ~(EFER_SCE | EFER_LME | EFER_LMA | EFER_NX | EFER_SVME |
-                  EFER_LMSLE | EFER_FFXSE) )
-        PRINTF("EFER: undefined bits are not zero (%#"PRIx64")\n", efer);
+    if ( efer & ~EFER_KNOWN_MASK )
+        PRINTF("EFER: unknown bits are not zero (%#"PRIx64")\n", efer);
 
     if ( hvm_efer_valid(v, efer, -1) )
         PRINTF("EFER: %s (%"PRIx64")\n", hvm_efer_valid(v, efer, -1), efer);
diff --git a/xen/arch/x86/pv/emul-priv-op.c b/xen/arch/x86/pv/emul-priv-op.c
index 15f42b3..ce2ec76 100644
--- a/xen/arch/x86/pv/emul-priv-op.c
+++ b/xen/arch/x86/pv/emul-priv-op.c
@@ -867,9 +867,16 @@ static int read_msr(unsigned int reg, uint64_t *val,
         return X86EMUL_OKAY;
 
     case MSR_EFER:
-        *val = read_efer();
+        /* Hide unknown bits, and unconditionally hide SVME from guests. */
+        *val = read_efer() & EFER_KNOWN_MASK & ~EFER_SVME;
+        /*
+         * Hide the 64-bit features from 32-bit guests.  SCE has
+         * vendor-dependent behaviour.
+         */
         if ( is_pv_32bit_domain(currd) )
-            *val &= ~(EFER_LME | EFER_LMA | EFER_LMSLE);
+            *val &= ~(EFER_LME | EFER_LMA | EFER_LMSLE |
+                      (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL
+                       ? EFER_SCE : 0));
         return X86EMUL_OKAY;
 
     case MSR_K7_FID_VID_CTL:
diff --git a/xen/include/asm-x86/msr-index.h b/xen/include/asm-x86/msr-index.h
index c9f44eb..6d94d65 100644
--- a/xen/include/asm-x86/msr-index.h
+++ b/xen/include/asm-x86/msr-index.h
@@ -31,6 +31,9 @@
 #define EFER_LMSLE             (1<<_EFER_LMSLE)
 #define EFER_FFXSE             (1<<_EFER_FFXSE)
 
+#define EFER_KNOWN_MASK                (EFER_SCE | EFER_LME | EFER_LMA | 
EFER_NX | \
+                                EFER_SVME | EFER_LMSLE | EFER_FFXSE)
+
 /* Speculation Controls. */
 #define MSR_SPEC_CTRL                  0x00000048
 #define SPEC_CTRL_IBRS                 (_AC(1, ULL) << 0)
-- 
2.1.4


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